Black’s thickness is imperfect because W “a” to cut him apart might (in some global positions) be feasible.
But even if that isn’t true, there still remains the annoying and potentially dangerous peeps at “b” or “c” to worry about.
And in the current position “d” is also open, so a later W“e” at the right time (with potential to connect underneath) will reduce the value of this Black formation by a major amount.
So the unavoidable conclusion is that as things stand Black’s thickness here is at best incomplete, and these Black stones are not strong!